The present invention relates to mounting a combustion chamber having a wall made of ceramic matrix composite (CMC) material inside a metal casing, in a gas turbine. The field of application of the invention is more particularly that of industrial gas turbines and of turbojets or turboprops for airplanes.
It is common practice for a gas turbine combustion chamber to be made of metal and to be mounted or secured inside a metal casing by linking members, ferrules or tabs, that are made of metal. Using a metal for the wall of the chamber is appropriate so long as it is possible to ensure effective cooling of said wall. However, there is a need to increase temperatures within the combustion chamber in order to increase the efficiency of the gas turbine and reduce polluting emissions. The use of metals for combustion chamber walls can then become inappropriate, even when implementing cooling as effectively as possible. Proposals have therefore been made for the walls of combustion chambers to be made out of ceramic matrix composite materials, such as composite materials having a silicon carbide (SiC) matrix and presenting good strength at high temperatures.
A problem which then arises is that of connecting the CMC combustion chamber to the metal casing, because of the differences between their coefficients of thermal expansion.
Document FR 2 825 783 proposes connecting the inner and outer annular walls of a CMC combustion chamber of a gas turbine to inner and outer metal shrouds of a metal casing by means of elastically-deformable metal linking tongues. Those metal tongues are secured at one end to a metal ferrule fastened to the inner or outer metal shroud, and at an opposite end to a CMC ferrule that is brazed onto the outside face of an inner or outer wall of the combustion chamber.
Accommodating the differential changes in dimensions between the combustion chamber and the metal casing is thus made possible by the flexible linking tongues having CMC-on-CMC connections at the combustion chamber end and metal-on-metal connections at the casing end. However, the brazed connection between the CMC ferrule and the annular wall of the combustion chamber leads to real difficulties. An effective brazed connection requires the spacing between the surfaces that are to be brazed together to be well controlled in order to guarantee a uniform thickness of brazing material and in order to avoid harmful discontinuities in the brazing. Unfortunately, given the processes whereby CMC parts are manufactured, the dimensional tolerances thereof are greater than is the case for metal parts. It is therefore very difficult to guarantee uniform spacing between two complete annular surfaces that are to be connected together by brazing.